1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method and apparatus for searching for a control channel by a user equipment (UE) in a wireless communication system.
2. Related Art
Recently, an amount of data transfer of a wireless communication network has rapidly increased. The increase in the amount of data transfer attributes to machine-to-machine (M2M) communication and the advent and prevalence of various devices such as smartphones, tablet PCs, and the like, requiring a large amount of data transfer. In order to meet the required large amount of data transfer, recently, carrier aggregation aimed at effective use of more frequency bands, cognitive radio technology, and multi-antenna technology and multi-base station cooperation technology aimed at increasing data capacity within a limited frequency, and the like, have emerged.
A wireless communication network has evolved toward increasing density of accessible nodes around users. Here, nodes may refer to antennas or antenna groups spaced apart from a distributed antenna system by a predetermined interval or greater, or without being limited thereto, but nodes may be used in a broader context. Namely, nodes may be a pico-cell base station (PeNB), a home base station (HeNB), a remote radio head (RRH), a remote radio unit (RRU), a repeater, and the like. A wireless communication system having high density of nodes may exhibit higher system performance through node cooperation. Namely, compared to a case in which nodes separately operate as a base station (BS), an advanced BS (ABS), a node-B (NB), an eNode-B (eNB), an access point (AP), or the like, without cooperation, if nodes operate like antennas or antenna groups with respect to a single cell under administration of a single control station in their transmission and reception, far better system performance may be attained. Hereinafter, a wireless communication system including a plurality of nodes will be referred to as a multi-node system.
Nodes generally refer to an antenna group spaced apart by more than a predetermined interval from a distributed antenna system (DAS). But nodes may also be defined as a certain antenna group regardless of a distance to the DAS. For example, a base station including cross polarized antennas may be considered as a base station including a node configured as an H-pol antenna and a node configured as a V-pol antenna, and the present invention may be applied thereto.
Meanwhile, in a multi-node system, a new control channel may be used due to an inter-cell interference and shortage of capacity in existing control channels. Existing control channels may be decoded based on a cell-specific reference signal (CRS), while the new control channel may be decoded based on a user-specific reference signal. Hereinafter, a new control channel will be referred to as an enhanced-PDCCH (E-PDCCH). Among a control region to which an existing PDCCH is allocated and a data region to which a data channel is allocated, the E-PDCCH may be allocated to the data region.
Two different resource allocation schemes such as non-interleaving and interleaving may be applied to the E-PDCCH region to which the E-PDCCH is allocated.
Meanwhile, in the multi-node system employing the E-PDCCH, a UE may be set to search for downlink control information (DCI) from only the E-PDCCH region. DCIs may include DCI with respect to a transmission (which is called a fallback mode transmission) to be performed just in case a UE fails to receive required control information or the control information is not reliable.
In which scheme a UE set to search for DCI from the E-PDCCH is to search for DCI for the fallback mode transmission is problematic.